A form of communication in which a radio terminal directly communicates with another radio terminal without communicating through an infrastructure network such as a base station is called device-to-device (D2D) communication. The D2D communication includes at least one of Direct Communication and Direct Discovery. In some implementations, a plurality of radio terminals supporting D2D communication form a D2D communication group autonomously or under the control of a network, and perform communication with another radio terminal in the formed D2D communication group.
Proximity-based services (ProSe) specified in the 3GPP Release 12 are examples of the D2D communication (see, for example, Non-patent Literature 1). ProSe direct discovery is performed through a procedure in which a radio terminal capable of performing ProSe (i.e., ProSe-enabled User Equipment (UE)) detects another ProSe-enabled UE by using only the capability of a radio communication technology (e.g., Evolved Universal Terrestrial Radio Access (E-UTRA) technology) possessed by these two UEs. ProSe direct discovery may be performed by three or more ProSe-enabled UEs.
ProSe direct communication enables establishment of a communication path between two or more ProSe-enabled UEs existing in a direct communication range after the ProSe discovery procedure is performed. In other words, ProSe direct communication enables a ProSe-enabled UE to directly communicate with another ProSe-enabled UE, without communicating through a Public Land Mobile Network (PLMN) including a base station (eNodeB). ProSe direct communication may be performed by using a radio communication technology that is also used to access a base station (eNodeB) (i.e., E-UTRA technology) or by using a Wireless Local Area Network (WLAN) radio technology (i.e., IEEE 802.11 radio technology).
In the 3GPP Release 12, a radio link between radio terminals used for direct communication or direct discovery is called a Sidelink (see, for example, Section 14 of Non-patent Literature 2). Sidelink transmission uses the Long Term Evolution (LTE) frame structure defined for an uplink and a downlink and uses a subset of uplink resources in frequency and time domains. A radio terminal (i.e., UE) performs sidelink transmission by using Single Carrier FDMA (Frequency Division Multiple Access) (SC-FDMA) similar to that for the uplink.
In 3GPP Release 12 ProSe, allocation of a radio resource for sidelink transmission to a UE is performed by a radio access network (e.g., Evolved Universal Terrestrial Radio Access Network (E-UTRAN)) (see, for example, Sections 23.10 and 23.11 of Non-patent Literature 3). A UE that has been permitted to perform sidelink transmission by a ProSe function performs ProSe direct discovery or ProSe direct communication by using a radio resource allocated by a radio access network node (e.g., eNodeB). Sections 23.10 and 23.11 of Non-patent Literature 3 describe details of allocation of a radio resource for sidelink communication to a UE.
Regarding ProSe direct communication, two resource allocation modes, i.e., scheduled resource allocation and autonomous resource selection are specified. In the scheduled resource allocation for ProSe direct communication, when a UE desires to perform sidelink transmission, the UE requests an eNodeB to allocate a radio resource for sidelink transmission and the eNodeB allocates resources for sidelink control and data to the UE. Specifically, the UE transmits to the eNodeB a scheduling request to request an uplink (UL) data transmission resource (i.e., Uplink Shared Channel (UL-SCH) resource) and then transmits a Sidelink Buffer Status Report (Sidelink BSR) to the eNodeB by using a UL data transmission resource allocated by an uplink grant (UL grant). The eNodeB determines sidelink transmission resources to be allocated to the UE based on the Sidelink BSR and transmits a sidelink grant (SL grant) to the UE.
The SL grant is defined as Downlink Control Information (DCI) format 5. The SL grant (i.e., DCI format 5) includes contents such as a Resource for PSCCH, Resource block assignment and hopping allocation, and a time resource pattern index. The Resource for PSCCH indicates radio resources for a sidelink control channel (i.e., Physical Sidelink Control Channel (PSCCH)). The Resource block assignment and hopping allocation is used to determine frequency resources, i.e., a set of subcarriers (resource blocks), for transmitting a sidelink data channel for data transmission on a sidelink (i.e., Physical Sidelink Shared Channel (PSSCH)). The Time resource pattern index is used to determine time resources, i.e., a set of subframes, for transmitting the PSSCH. Note that, strictly speaking, the resource block means time-frequency resources in LTE and LTE-Advanced and is a unit of resources specified by consecutive OFDM (or SC-FDMA) symbols in the time domain and consecutive subcarriers in the frequency domain. In the case of Normal cyclic prefix, one resource block includes 12 consecutive OFDM (or SC-FDMA) symbols in the time domain and 12 subcarriers in the frequency domain. That is, the Resource block assignment and hopping allocation and the Time resource pattern index designate a resource block for transmitting the PSSCH. The UE (i.e., a sidelink transmitting terminal) determines a PSCCH resource and a PSSCH resource according to the SL grant.
Meanwhile, in the autonomous resource selection of ProSe direct communication, a UE autonomously selects resources for sidelink control (i.e., PSCCH) and data (i.e., PSSCH) from a resource pool(s) set by an eNodeB. The eNodeB may allocate a resource pool(s) for the autonomous resource selection to the UE in a System Information Block (SIB) 18. The eNodeB may allocate a resource pool for the autonomous resource selection to the UE in Radio Resource Control (RRC)_CONNECTED via dedicated RRC signaling. This resource pool may be available when the UE is in RRC_IDLE.
Regarding ProSe direct discovery, two resource allocation modes, i.e., scheduled resource allocation and autonomous resource selection are also specified. In the autonomous resource selection for ProSe direct discovery, a UE that desires transmission (announcement) of a discovery signal (discovery information) autonomously selects radio resources from a resource pool(s) for announcement. This resource pool is configured in UEs via broadcast (SIB 19) or dedicated signaling (RRC signaling).
Meanwhile, in the scheduled resource allocation for ProSe direct discovery, a UE requests an eNodeB to allocate resources for announcement via RRC signaling. The eNodeB allocates resources for announcement from a resource pool that is configured in UEs for monitoring. When the scheduled resource allocation is used, the eNodeB indicates in SIB 19 that it provides resources for monitoring of ProSe direct discovery but does not provide resources for announcement.
When direct transmission is performed on a sidelink, a UE on a transmitting side (i.e., a D2D transmitting UE) (hereinafter referred to as a transmitting terminal) transmits Scheduling Assignment information by using a portion of radio resources (i.e., resource pool) for a sidelink control channel (i.e., PSCCH). The scheduling assignment information is also referred to as Sidelink Control Information (SCI) format 0. The scheduling assignment information includes contents such as resource block assignment and hopping allocation, a time resource pattern index, and a Modulation and Coding Scheme (MCS). In the case of the above-described scheduled resource allocation, the Resource block assignment and hopping allocation and the time resource pattern index indicated by the Scheduling Assignment (i.e., SCI format 0) follow the Resource block assignment and hopping allocation and the time resource pattern index indicated by the SL grant (i.e., DCI format 5) received from the eNodeB.
The transmitting terminal transmits data on the PSSCH by using a radio resource according to the scheduling assignment information. A UE on a receiving side (i.e., a D2D receiving UE) (hereinafter referred to as a receiving terminal) receives the scheduling assignment information from the transmitting terminal on the PSCCH and receives the data on the PSSCH according to the received scheduling assignment information. Note that the term “transmitting terminal” just focuses on a transmission operation of a radio terminal and does not mean a radio terminal used for transmission only. Similarly, the term “receiving terminal” is an expression for expressing a receiving operation of a radio terminal and does not mean a radio terminal used for reception only. That is, the transmitting terminal is able to perform a receiving operation and the receiving terminal is able to perform a transmitting operation.